The Oppenauer oxidation (Op-Ox) and the inverse reduction, the Meerwein-Pondorf-Verley reduction (MPV-Red), are well-established oxidation or reduction processes for alcohols or ketones, respectively. The reactions are possible without requiring toxic heavy metals as catalysts (see, for example, J. March, Advanced Organic Chemistry, 3rd Ed., John Wiley 1985, pp 1058, 813; ISBN 04718547-7; C. F. deGraauw, J. A. Peters, H. van Bekkum, J. Huskens, Synthesis, 1994, 1007-17; and S. D. Burke, D. L. Danheiser, Handbook of Reagents for Organic Synthesis, Oxidizing And Reducing Reagents, Wiley 1994; ISBN 0471979260.
A disadvantage is that the strongly basic reaction conditions lead to undesired aldol-type side reactions (see K. G. Akamanchi, B. A. Chaudhari, Tetrahedron Lett. 38, 6925-8 (1997)). Substrates sensitive to basic conditions cannot be reacted without decomposition (see T. Ooi, Y. Itagaki, T. Miura, K. Maruoka, Tetrahedron. Lett. 40, 2137-8 (1999)). Asymmetric variants of the MPV-Red that employ chiral transition metal catalysts for enantioselective hydrid transfer have been tested only on few model substrates and resulted in preparatively inacceptably low stereoselectivities (see F. Touchard, M. Bernard, F. Fache, F. Debbecq, V. Guiral, P. Sautet, M. Lemaire, J. Org. Met. Chem. 567, 133-6 (1998); und E. Breysse, C. Pinel, M. Lemaire, Tetrahedron: Asymmetry 9, 897-900 (1998)).
In contrast, biocatalytic methods have the advantage that they can be led under mild conditions, e.g. at room temperature and approximately neutral pH in aqueous media (see K. Faber, Biotransformations in Organic Chemistry 4th Ed., Springer Verlag., Heidelberg 2000; ISBN 3-540-61688-8). An additional valuable property of biocatalysts is their normally high intrinsic stereoselectivity. In addition, the desired reaction usually takes place without side reactions. Biocatalytic redox processes on the basis of isolated alcohol dehydrogenases, however, require the presence of expensive cofactors, such as NAD+/NADH or NADP+/NADPH. The recycling of these substrates is difficulty and expensive (see W. Hummel, Adv. Biochem. Eng./Biotechnol. 1997, 58, 145-184). One improvement of such processes is based on the presence of a second enzyme which, in the presence of a reducible or oxidizable component or cofactor, respectively, allows to recover the co-factor (“enzyme-coupled system”, see W. Hummel, B. Riebel, Ann. N. Y. Acad. Sci. 1996, 799, 713-6). This variant, however, renders the process relatively complex and difficult to handle, as it is limited to such additives which are accepted as auxiliary component by the second enzyme. In addition, the concentrations of the substrates and the enzymes must be harmonized precisely for resulting in preparatively acceptable reaction rates. Furthermore, isolated enzymes usually have relatively short half lives under operating conditions. In order to achieve complete reaction into one direction, in the case of oxidation a carbonyl compound is added as reducible co-substrate in large molar excess, in the case of reduction a secondary alcohol as oxidizable co-substrate is added in large molar excess. This often results in difficulties especially with enzyme stability, as well as enzyme inhibition by the co-substrate.
If whole cells are used as biocatalysts in the stadium of fermentation, a lower addition of co-factors is required. In addition the cells are capable of recycling the co-factors themselves. However, the cells react very sensitively on high concentrations of organic substrates (substrate inhibition, solvent deactivation). For this reason, the biochemical MPV-Red and Op-Ox are limited to fermentative cells systems and low (co-)substrate concentrations (see G. Fantin, M. Fogagnolo, A. Medici, P. Pedrini, S. Fontana, Tetrahedron: Asymmetry 2000, 11, 2367-73). In general, the substrate concentrations are below 0.15 mol/l and the co-substrate concentrations below 3% (v/v) (see K. Nakamura, Y. Inoue, T. Matsuda, I. Misawa, J. Chem. Soc. Perkin Trans. I 1999, 2397-2402; and A. Goswami, R. L. Bezbaruah, J. Goswami, N. Borthakur, D. Dey, A. K. Hzarika, Tetrahedron: Asymmetry 2000, 11, 3701-9).
Reductions of ketones to the respective alcohols have also been conducted with acetone-pulverized Geotrichum candidum cells (see K. Nakamura, T. Matsuda, J. Org. Chem. 1998, 63, 8957-64). However, in this case, the cellular redox systens was partially inactivated during the freeze-drying of the cells. This required the addition of expensive redox-cofactors, such as NAD+/NADH or NADP+/NADPH, for subsequent use as biocatalyst. The addition of (reducing) isopropyl alcohol as co-substrate was limited to about 3% (v/v) in that case. Due to the relatively low concentration of this organic co-solvent, the concentration of lipophilic ketone substrates could only be adjusted to a maximum of 0.4 mol/l.
In view of all these unsatisfying results which constitute an unmet need, there remains the problem to find novel, more efficient catalytic systems offering more ease of use, which diminish or remove the mentioned and other unfavorable disadvantages and allow to conduct redox reactions of alcohols to oxo (especially keto) compounds or the inverse reaction in an especially advantageous way.
It is thus an object of the present invention to provide novel catalytic systems that allow for avoiding the mentioned disadvantages and that have advantageous properties that allow for improved biocatalytic reduction of ketones and/or oxidation of secondary alcohols, respectively.